Test Equipment: Oscillocopes

[poynt99]

PW, 

[picowatt]

I would strongly advise against that.  If there are little trim pots all over the boards in your scope that typically means that the boards had to be tuned and aligned with the trimpots as part of the manufacturing process.  Changing a cap could adversely affect the tuning and alignment of your scope.  I am being cautious and very generic in my statement.

And you don't want to take a chance of cracking or pulling out a plated thru hole or lifting a pad if you don't have to. 

Brian said he is blowing out the solder (I assume with compressed air or Dust off) which is OK if you know where the solder is going!  Don't let it blow onto a board somewhere where it will short out something causing another problem.

When solder sucking or blowing out plated thru holes, it may be necessary to resolder the lead and do it again if you don't see daylight around the lead the first time.  Resoldering allows the joint to wet again.  Also, it is always a good idea to snap the lead loose from the side of the plated thru hole using a small flat bladed screwdriver or small needle nose plier and sideways force.  Resist the temptation to pull on the part until you can easily wiggle all leads, otherwise you may pull out a small copper cylinder that was once the thru hole plating.  Sometimes it is best just to carefully cut the part's leads first and then pull each lead out of the board one at a time while applying heat.  After all the leads are out, the hole can be cleared of solder (via sucking, blowing, or wicking).  This is particularly advisable with leads that are a tight fit in the PCB hole that do not allow sucking out the solder all that well. 

PW

[MileHigh]

PW:

Funny story.  That repair tech guy had a fail.  My first thought was to take my variable bench power supply and inject some current and put a few watts in and then do a "laying on of hands" to find the cap acting like a toasty resistor.  Your way was quite dramatic.

My semi-related story was running a boom box off a Radio Shack power inverter circa 1980.  This was in a big private bus going down the highway.  We heard this incredibly loud boom.  It was so loud that we all thought a tire blew out.  There was a big non-polarized electrolytic cap (I think) in the inverter that blew up.

Yes about those tants.  They look like mysterious gooey stuff must be encapsulated in there.  A dangerous thing is inserting radial tantalum caps manually into boards on a push line.  A very nice lady in a blue frock may make a mistake and it's had to detect.  I don't think a reversed-biased tantalum cap turns into a gooey mess right away when it is wrongly inserted so it can make it into a shipping box.

MileHigh

[picowatt]

PW:

Funny story.  That repair tech guy had a fail.  My first thought was to take my variable bench power supply and inject some current and put a few watts in and then do a "laying on of hands" to find the cap acting like a toasty resistor.  Your way was quite dramatic.

My semi-related story was running a boom box off a Radio Shack power inverter circa 1980.  This was in a big private bus going down the highway.  We heard this incredibly loud boom.  It was so loud that we all thought a tire blew out.  There was a big non-polarized electrolytic cap (I think) in the inverter that blew up.

Yes about those tants.  They look like mysterious gooey stuff must be encapsulated in there.  A dangerous thing is inserting radial tantalum caps manually into boards on a push line.  A very nice lady in a blue frock may make a mistake and it's had to detect.  I don't think a reversed-biased tantalum cap turns into a gooey mess right away when it is wrongly inserted so it can make it into a shipping box.

MileHigh

MH,

With the supply folded back, nothing was getting all that warm (including the supply).

And yes, a backwards installed tant tends to first get a bit warm, then get very noisey, and then finally will usually short.  If they are of sufficiently high V rating relative to the rail, they can, as you say, survive long enough to QC the equipment and get it out the door. 

And then, of course, there is infant mortality...

PW

[TinselKoala]

@MH and PW: I suggest you look at the videos Brian has posted. The scope is a Tek 465 and the SM can be downloaded from the link I posted earlier in the thread, so that you can be "on the same page" as we are, so to speak.

I've located the caps in question on the parts list and on the schematics in that version of the SM. The questionable C1476 cap referred to is indeed listed as a ceramic and the photo listing in the parts source that Brian found looks very much like the cap photo on the board. It is a multilayer ceramic, and its position in the circuit leads me to think that it could indeed be causing at least one of the glitches that he has shown in the videos.

All help is of course appreciated, and I'm especially glad that PW is interested. He may be able to tell much more from the schematic and the behaviour as shown in the  video than I can.

Those old doublesided circuit boards are usually very robust. PW's advice about clipping the component leads off very short is a good one, and then the remainder can be pushed out with a wooden toothpick while heating the pad with the iron. This process is actually recommended by Tek for the plated thru-holes, to clear them for reinstallation of a new component.

Making sure that the removed solder doesn't get on anything is a rather elementary caution.

@Brian: the board images that I see: some of them are quite readable but others are not. I really don't think it's a problem with my software or the use of it.